A type of an image detector for radiodiagnosis as currently used is configured to detect an irradiated X rays and thus acquire an X-ray radiograph as a digital signal. This kind of radiation detector can roughly be divided into a direct X-ray detect and/or an indirect X-ray detector. An indirect X-ray detector acquires an image by changing an X ray into visible light by a phosph and/or causing a photoelectric converter such as an amorphous silicon (a-Si) photodiode, single-crystal silicon (c-Si) photodiode, and charge couple device (CCD) to convert the visible light into a charge signal.
When a-Si is used as the photoelectric converter of the indirect X-ray detector, since amorphous silicon has sensitivity in the wavelength band region of 450 nm to 650 nm, the phosphor is required to show luminescence in the wavelength band region of 450 nm to 650 nm. When c-Si is used as the photoelectric converter, since c-Si has sensitivity in the region of a longer wavelength than a-Si, the phosphor is required to show luminescence in the corresponding wavelength band region.
While high luminescence intensity is very important as one of capabilities demanded for a phosphor material, as another aspect, stability for the environment is also an important requirement. More specifically, the stability means characteristics superior in resistivity to deliquescence in which the phosphor material is not dissolved or hardly dissolved by water vapor in the air.
As described in PTL 1, superiority in resistivity to deliquescence of a scintillator material including caesium iodide and copper iodide mixed crystal has been known. PTL 1 describes that the structure represented as Cs3Cu2I5 is verified and the scintillator material including caesium iodide and copper iodide mixed crystal emits light having photon energy of 2.8 eV. The mixed crystal has a different crystal structure from that of caesium iodide.
Also as described in PTL 2, it is known that moisture resistance is improved by using a raw material obtained by adding 0.01 mol % of copper iodide and thallium iodide or more as additives to caesium iodide.